1. Field of the Invention
The present invention relates to a decision system for modulated electrical signals.
To be more precise, it relates to a decision system for use in receivers for optical systems for transmitting digital signals. A system of this kind is adapted to process a modulated electrical signal obtained by converting a received modulated optical signal. The electrical signal is delivered by a converter and has an amplitude varying in response to the amplitude of the optical signal and the system comprises means for comparing the amplitude of the electrical signal to a reference level.
2. Description of the Prior Art
It is standard practice to exchange streams of optical data in continuous mode, known as continuous streams, in which information is transmitted by a signal which is continuously modulated. However, this transmission mode necessitates reshaping of the signals and leads to a loss of compactness.
To obtain an improvement in terms of shaping and compactness, and to enable all-optical switching, and therefore faster switching, at the switching nodes, exchanging data in “packet” mode has been envisaged.
When streams of optical data are exchanged in “packet” mode, the data stream consists of a large number of packets spaced from each other in time, each packet containing information necessary for sending the packet to its destination and information relating to the characteristics of the data, for example computer data, voice data, etc.
The packets received by a receiver can come from different sources and can have been routed via different optical paths, as a result of which the individual packets reaching a receiver may have different optical power levels, sometimes very different optical power levels.
When a continuous stream optical signal is converted into an electrical signal, it is standard practice to use an electrical decision system connected to a bistable to ensure fast and correct interpretation of the signal. A typical reference level is a threshold value equal to 50% of the maximum amplitude of the electrical pulses. The decision system then includes means for comparing the amplitude of the electrical signal to the reference level.
Moreover, to obtain the best bit error rate (BER), the reference level in an electrical decision system in the receiver must be optimized as a function of the degradation of the signal caused by its transmission and switching in the optical system. As a general rule, there is more noise at the higher levels of the pulses constituting digital optical signals, and a pulse of the signal is therefore considered to be a 1 if it exceeds a reference level sufficiently below the maximum amplitude. In this situation the reference level can be a fixed optimum value corresponding to a threshold 50% less than the maximum amplitude of the optical pulses.
In “packet” mode transmission the amplitude of the optical pulses, and therefore that of the electrical pulses, can vary greatly from one packet to another, and there arises the as yet unsolved problem of determining a reference level and producing a decision system that can be applied to this mode of transmission.